The United States and European Union called on Syrian President Bashar al-Assad to step down on Thursday and President Barack Obama accused him of 'torturing and slaughtering' his people in what U.N. officials said would be crimes against humanity.

It was a dramatic sharpening of international rhetoric - major states had urged Assad to reform rather than resign.

But with no threat of Western military action like that against Libya's Muammar Gaddafi, the five-month-old conflict between Assad and his opponents seems likely to grind on in the streets.

Putting faith in sanctions rather than force, Obama ordered Syrian government assets in the United States frozen, banned U.S. citizens from operating in or investing in Syria and prohibited U.S. imports of Syrian oil products.

Though U.N. Secretary-General Ban Ki-moon said Assad had assured him Wednesday that military operations were over, activists said Syrian forces carried out further raids in Deir al-Zor and surrounded a mosque in Latakia Thursday.

"The future of Syria must be determined by its people, but President Bashar al-Assad is standing in their way," Obama said. "His calls for dialogue and reform have rung hollow while he is imprisoning, torturing and slaughtering his own people."

In a coordinated move, European Union foreign policy chief Catherine Ashton called on Assad to step aside and said the EU was preparing to broaden sanctions against Syria.

At the United Nations, Britain, France, Portugal and Germany said they would begin drafting a Security Council sanctions resolution on Syria. "We believe that the time has come for the council to take further action," Britain's Deputy U.N. Ambassador Philip Parham told reporters.

U.N. human rights investigators said Assad's forces had carried out systematic attacks on civilians, often opening fire at close range and without warning, and committing violations that may amount to crimes against humanity.

A U.N. report recounted complaints of indiscriminate shooting and of wounded people being put to death with knives or by being dumped in the refrigerated rooms of hospital morgues.

CRACKDOWN 'CONTINUING'

In a telephone call with Assad Wednesday U.N. Secretary General Ban joined a chorus of condemnation, expressing alarm at reports of widespread violations of human rights and excessive use of force by security forces against civilians.

The Syrian Revolution Coordinating Union, an activists' group, said security forces fired machineguns near a mosque in Latakia which was surrounded by armored vehicles.

It also said Assad's forces killed at least one man when they fired live ammunition to stop residents from marching after Ramadan prayers, known as tarawih, in the Mureijeh neighborhood of Homs, 165 km (100 miles) north of the capital Damascus.

Separately, it said security forces shot dead a man it identified as Ali al-Hussein and wounded six when they fired at a sit-in in the town of al-Ruhaibeh northeast of Damascus.

Similar attacks occurred in the Houla Plain north of Homs and in the town of Qusair on the Lebanese border to the southwest, but there were no immediate reports of casualties.

U.N. INVESTIGATION

The U.N. investigators said Syrian forces had fired on peaceful protesters throughout the country, often at short range and without warning, killing at least 1,900 civilians, including children. Their wounds were "consistent with an apparent shoot-to-kill policy," their report said.

Some were reported to have been finished off with knives.

"The mission found a pattern of human rights violations that constitutes widespread or systematic attacks against the civilian population, which may amount to crimes against humanity," it said, specifically citing the Rome Statutes of the International Criminal Court (ICC).

There was a "clear pattern of snipers shooting at demonstrators," and in some cases targeting people trying to evacuate the wounded. In hospitals "there were several reports of security forces killing injured victims by putting them alive in refrigerators in hospital morgues."

The United Nations also plans to send a team to Syria this weekend to assess the humanitarian situation there, a U.N. official said Thursday.